1. Field of the Invention
The present invention relates to a magnetic recording head, a method of manufacturing the same, and a magnetic disk unit, and particularly relates to a magnetic recording head having a spin torque oscillator of a microwave-assisted magnetic recording head system, a method of manufacturing the same, and a magnetic disk unit including the magnetic recording head.
2. Description of the Related Art
In recent years, magnetic recording heads supporting higher recording densities in hard disk drives (HDDs) have been reduced in size, leading to technical difficulty in obtaining a sufficient magnetic field for flux reversal. As a solution to this problem, a technique called energy-assisted recording has received attention. In energy-assisted recording, some energy is applied to a medium to reduce a necessary magnetic field for flux reversal, so that a magnetic signal can be written to a recording medium that is unrecordable under normal conditions. Thus, higher recording capability and smaller recording regions can be achieved. Energy used for assisted recording is Joule heat generated by a laser (thermally assisted recording). Such recording is a representative energy-assisted recording mode.
Moreover, in a known technique, a high-frequency field is used as energy for assisting recording. This technique is a promising technique of energy-assisted recording. For example, in Japanese Patent Laid-Open No. 6-243527 have proposed a microwave-assisted magnetic recording system that facilitates writing by applying an assist magnetic field of GHz microwaves to a recording medium. In ‘Microwave Assisted Magnetic Recording,’ The Magnetic Recording Conference (TMRC) 2007 Paper B6 (2007), J. G. Zhu and X. Zhu, a technique is reported in which a magnetization high-speed rotor (Field Generation Layer: FGL) for high-speed magnetization rotation by a spin torque is disposed near a magnetic recording medium next to the main pole of a perpendicular magnetic head to generate microwaves, so that information is recorded on a magnetic recording medium having large magnetic anisotropy. Furthermore, in “Microwave-assisted magnetization reversal in a Co/Pd multilayer with perpendicular magnetic anisotropy. Applied Physics Letters. 2009, vol. 95, p. 082505-1-3.”, Nozaki, Y. et al., a flux reversal assist is reported that uses a Co/Pd artificial lattice film having perpendicular magnetic anisotropy. A material used in this technique has also been studied as a medium material for perpendicular magnetic recording, which proves that microwave-assisted magnetic recording is effectively applicable to a perpendicular magnetic recording medium.
As reported above, microwave-assisted magnetic recording with spin torque oscillators has been currently promoted.
In microwave-assisted magnetic recording, information is recorded on a medium by combining a recording magnetic field generated from a main pole and an assist magnetic field generated from a spin torque oscillator, so that a track width written on the medium depends on the width of an FGL layer in the spin torque oscillator. Hence, it can be said that micro fabrication of spin torque oscillators is a significant technique for improving a recording density.
Micromachining of spin torque oscillators has, however, the following technical problems:
A first problem is an offset (misalignment) between a main pole and a spin torque oscillator. Machining of a spin torque oscillator requires the formation of a Stripe Height direction and a track width direction. In such machining, etching such as milling and techniques such as photolithography are used. Photolithography has been recently developed in which a fine pattern of several tens nm can be exposed with a photo deviation of ±20 nm. However, it is virtually impossible to eliminate such a photo deviation, so that a fine pattern is considerably affected by a photo deviation. Also in machining of a spin torque oscillator, such a photo deviation is not negligible in the formation of a spin torque oscillator having a width of several tens nm on a main pole that is a pattern having a width of several tens nm.
Even in the case where the deviation of a spin torque oscillator is only 10 nm from the center of a main pole width, an assist effect is considerably degraded. Since a trailing shield and a main pole are used as electrodes, a large deviation of the spin torque oscillator causes a displacement of a contact, so that an electrical resistance may increase. In other words, unless the influence of a photo deviation is eliminated in the formation of the spin torque oscillator, it is quite difficult to stably obtain a desired shape.
In the age of in-plane magnetic heads, self alignment was generally used to align the widths of an upper magnetic pole and a lower magnetic pole. In the technique of Japanese Patent Laid-Open No. 11-161915, when a lower magnetic pole is trimmed to an upper magnetic pole, the widths of the magnetic poles can be aligned by a protective film provided on the side wall of the upper magnetic pole. In the case where a main pole and a spin torque oscillator are formed by self alignment in microwave-assisted recording, the upper magnetic pole and the lower magnetic pole of an in-plane head are covered only with an insulating film, whereas the spin torque oscillator and the main pole are covered with a magnetic film. Because of this difference, self alignment is not similarly effective.
A second problem is etching damage on a spin torque oscillator including a laminated film. The problem of an offset may be solved by collectively etching (self alignment) a main pole and a spin torque oscillator by techniques such as milling with a single etching mask. In the case of collective formation by physical etching such as milling, however, an etched material of the main pole may adhere to the end of the spin torque oscillator. Moreover, etching damage or edge roughness may occur on the spin torque oscillator. Thus, only such collective formation may cause damage on the spin torque oscillator.
A third problem is the locations of side shields.
In perpendicular magnetic recording, a phenomenon called “blurred writing” has been a problem caused by a spatial expansion of a recording magnetic field generated from a main pole. As an effective solution, a magnetic material shield disposed around a main pole is known. Also in the case of a spin torque oscillator, it is effective to dispose side shields around the spin torque oscillator so as to concentrate an assist magnetic field around a desired recording bit. A gap film is disposed between the shield and the main pole or the spin torque oscillator to reduce magnetic interaction. The spacing (corresponding to a gap thickness) between the shield and the main pole or the spin torque oscillator is an important controllable factor for the performance of the shield, the main pole, and the spin torque oscillator. This is because in the case of an extremely thin gap film, a generated recording magnetic field or assist magnetic field is absorbed by the shield, precluding the generation of a desired magnetic field on a magnetic recording medium. In the case of an extremely thick gap film, the effect of suppressing the spatial expansion of a recording magnetic field or assist magnetic field may be lessened. Furthermore, it is necessary to consider the influence of a change of shield magnetization, which may be changed by a signal magnetic field from a magnetic recording medium and a recording magnetic field from the main pole. In the case of a spin torque oscillator having an extremely thin gap film, a shield magnetization process may adversely affect the oscillation characteristics of the spin torque oscillator, degrading a desired recording assist effect.
In the technique of Japanese Patent Laid-Open No. 2010-3351, a magnetic film is provided between a spin torque oscillator and a third magnetic layer. It is considered that the magnetic film does not prevent blurred writing unlike side shields but acts as a bypass for dispersing magnetic flux passing through the spin torque oscillator from a main pole. Furthermore, the widths of the main pole and the spin torque oscillator are not described in this technique and thus the problem of this technique is different from the third problem.